With the LC using the multi-channel detector such as the PDA detector, it is possible to obtain three-dimensional chromatogram data having three dimensions, namely, time, wavelength, and absorbance (signal intensity), by repeatedly obtaining an absorption spectrum over a predetermined wavelength range, from a sample liquid that flows out from a column outlet, while setting a point in time of injection of a sample to a mobile phase as a base point. With a liquid chromatograph mass spectrometer (LC-MS) or a gas chromatograph mass spectrometer (GC-MS), it is possible to obtain three-dimensional chromatogram data having three dimensions, namely, time, mass-to-charge ratio, and signal intensity (ion intensity), by repeating a scan measurement over a predetermined mass-to-charge ratio range using the mass spectrometer.
Portion (a) of FIG. 2 is a conceptual diagram of three-dimensional chromatogram data obtained by the LC described above. Through extraction of absorbance data in a time direction at a specific wavelength (for example, a wavelength λ0) from such three-dimensional chromatogram data, a chromatogram at the wavelength λ0 (hereinafter, referred to as a “wavelength chromatogram”) as illustrated in Portion (b) of FIG. 2 can be created. Further, through extraction of absorbance data in a wavelength direction at a specific measurement time (for example, time tp) from the three-dimensional chromatogram an absorption spectrum at the specific measurement time tp can be created.
To carry our quantitative determination of a known compound included in a sample on the basis of such chromatogram data, a wavelength chromatogram at an absorption wavelength at which the target compound best absorbs light is typically created. The quantitative value is calculated by finding a peak derived from the target compound on the wavelength chromatogram, calculating an area value of the peak, and comparing the area value to a calibration curve obtained in advance. For this reason, to achieve accurate quantitative determination, it is important to accurately calculate the area value of the peak corresponding to the target compound on the chromatogram.
However, in general, a baseline derived from a mobile phase and the like is present on the wavelength chromatogram. Moreover, the peak derived from the target compound is sometimes overlapped with a peak derived from another compound. Therefore, in order to correctly calculate the peak area value corresponding to the target compound, it is necessary to find a true peak region excluding the influence of a baseline derived from the mobile phase and the like through correct estimation of the baseline.
A data processing device generally used in the LC system is provided with a peak waveform processing function that automatically estimates a baseline on the basis of a chromatogram waveform obtained by measurement. However, depending on the shape of the chromatogram waveform, it is not possible to estimate a suitable baseline by the automatic waveform processing in many cases. To address this problem, as disclosed in Non-Patent Document 1, the data processing device of the known art is configured such that a user can estimate a more suitable baseline by manually changing or setting, as appropriate, the parameter of the waveform processing and the algorithm for the waveform processing to be applied.
FIG. 8 illustrates exemplary baselines and regions in which an area value of a peak P is to be calculated (region indicated by hatched lines in FIG. 8), wherein the baselines and the peak P are estimated from a chromatogram waveform, based on the waveform processing disclosed in Non-Patent Document 1. In FIG. 8, portion (a) is an example in which the peak is vertically divided by setting a signal zero level as the baseline, after removing negative peaks. Portion (b) is an example in which the bottoms of the peaks are connected by a straight line to form the baseline. Portion (c) is an example in which the bottoms of the peaks are connected by a curved line to form the baseline. From these examples, it is evident that the peak area values differ greatly depending on how the baseline is drawn.
In this respect, for example, when a user determines that the baseline drawn by the waveform processing based on a certain algorithm is not suitable, the user can calculate a more suitable baseline and improve the accuracy of the peak area value by selecting another waveform processing which uses a different algorithm. However, in this case, the user him/herself (for example, an operator who is responsible for the analysis) needs to determine whether to connect the bottoms of the adjacent peaks with a straight line or a curved line, or which portion is to be regarded as the base of the peak derived from the target compound when the base of the peak seems to be elongated. To make such a determination accurately, the operator needs to have a certain degree of experience or skills on the peak waveform processing. Moreover, the determination may differ depending on the operator in charge. As a result, the shape of the peak chromatogram waveform from which the baseline is removed, and the peak area value may vary even though the original chromatogram is the same.
The baseline present in the wavelength chromatogram at each wavelength influences the waveform shape of the absorption spectrum. Consequently, to identify the compound on the basis of the position of the peak appearing in the absorption spectrum (in other words, an absorption wavelength), the baseline needs to be suitably estimated for each of wavelength chromatograms at wavelengths different from one another. However, since an enormous number of wavelength chromatograms are obtained by a single measurement, it is practically impossible for a user to carry out the baseline estimation process for each of the wavelength chromatograms while manually setting a parameter and the like for each wavelength chromatogram.